Heirarchical name space allocation /Trees

Christian Maeder maeder at tzi.de
Thu Apr 1 13:00:59 EST 2004

Ross Paterson wrote:
> All the usual advantages of abstraction.
> I thought SizedSeq was pretty compelling:
> 	data Sized s a = N !Int (s a)
> 	instance Sequence s => Sequence (Sized s) where ...
> One instance gets you N new sequences.

The disadvantage for the user is that for a "Sized Seq" also "Seq" will 
be in scope and changing the implementation either requires to rename 
types, to use clever type synonyms, or to use the "Sequence" class 

However, I would not mind such a class if it eases other implementions, 
but from a user's point of view, I want different modules with the same 
interface (and as few as possible class constraints).

"data Sized" is of course generally useful (not only for Sequence) and 
may be accompanied by a couple of wrap and unwrap functions for certain 
function profiles to be lifted.


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